x/build: set up builders simulating old processors

[bradfitz]

We occasionally get a bug report from users that some assembly code (often in runtime or crypto or compression) causes a SIGILL.

We should run builders (perhaps in qemu?) simulating older processors so we can exercise all the assembly fallback paths and CPU detection code.

[bradfitz]

As a starting build, @saljam made a qemu builder in the past:

https://github.com/golang/build/tree/master/env/linux-arm-qemu

Using it as a model, an x86-on-x86 builder would be interesting, masking away certain CPU features.

[josharian]

This would also be helpful for a theoretical future RISC-V builder.